People often ask this and the answer is generally not, unless the system is an Archimedean Screw because you would only see a benefit if the head varied a lot at the site.
This is because if the head varies the velocity of the water through the turbine varies, and if the turbine blades are to operate at maximum efficiency the rotational speed of the turbine must be adjusted slightly to compensate. For many sites head variations are not significant enough to be an issue, though they can be on very low-head sites where the downstream water level can increase during higher flow periods. Even then, the impact needs to be significant because the extra benefits from variable-speed operation have to outweigh the extra electrical losses in the generator – inverter/rectifier – inverter chain that is required, and also the additional capital costs of the equipment.
Variable speed generator use for wind systems are common, but this is because the equivalent of the head varying in hydro for wind is the wind speed varying, which clearly happens constantly. Therefore wind turbines get a significant performance benefit from variable-speed operation.
The only caveat required is Archimedean Screws. Archimedean Screws are ‘gravity machines’ like overshot and breastshot waterwheels, so there is no need to align a turbine blade to the oncoming flow at an optimum angle. However, they do have to be able to vary the flow rate through themselves which is most efficiently achieved by varying the rotational speed of the screw, hence making it a variable-speed screw. The alternative is a fixed-speed screw with an inlet sluice gate which raises and lowers to adjust the flow rate through the screw, but this creates significant head losses, particularly at low flows. A secondary but significant benefit of variable-speed screws is that they do not suffer from the ‘back-slapping’ noise that is common to fixed-speed screws and in bad cases has led to noise abatement orders being served. For these reasons Renewables First would always recommend using a variable-speed Archimedean Screw.
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